Emulsion polymerization in an aqueous medium is utilized for the production of various polymers because of freedom from the danger of fire and environmental pollution due to the unnecessity of any organic solvent. In the production of an acrylic polymer by emulsion polymerization, ammonium persulfate is frequently used as a water-soluble radical polymerization initiator in an amount of usually from 0.1 to 0.5 parts by weight per 100 parts by weight of the monomer, and the polymerization is conducted usually at a temperature of 60.degree. C. or higher, with the decomposition temperature of the initiator being taken into account. In the case of mass production, the monomer-dropping method in which a monomer emulsion is dropped into the polymerization system little by little is generally employed to conduct emulsion polymerization because feeding a monomer at a time in mass production makes the control of heat generation by polymerization difficult.
The acrylic polymer produced by such conventional emulsion polymerization contains a considerable proportion of a low molecular weight component. The presence of this low molecular weight component seems to be desirable because it enhances tack in pressure-sensitive applications such as adhesive tapes. However, where the polymer is used over a prolonged period of time in a high temperature range or under high load or used in applications where an end-peeling force is applied or a plasticizer is present, the adhesiveness of the polymer to an adherend often becomes poor. This is because the low molecular weight component blooms even though the cohesive force of the polymer has been increased by post-crosslinking alone. In re-strippable applications, the conventional acrylic polymer also has a problem that the low molecular weight component blooms on the surface of the adherend to stain the adherend upon re-stripping.
Because of the above, polymers for the above adhesive applications preferably have a low molecular weight component content which is as low as possible. However, diminishing the low molecular weight component in a polymer is never easy, although increasing the low molecular weight component can be easily attained by increasing the amount of a chain transfer agent. A reduction of the content of the low molecular weight component seems to be attained by reducing the amount of ammonium persulfate used as a polymerization initiator to below the lower limit of the above specified range or by lowering the polymerization temperature to below 60.degree. C. However, the polymerization reaction is almost stopped under such conditions, so-that the desired acrylic polymer is difficult to produce.
Another method which seems to be usable to obtain the desired polymer is to remove the low molecular weight component after emulsion polymerization in place of inhibiting the formation of a low molecular weight component during emulsion polymerization. For example, polymer reprecipitation and cleaning the pressure-sensitive adhesive on an adhesive tape with an organic solvent are known. However, such techniques are costly and laborious and are hence undesirable for practical use.
As described above, a process for industrially advantageously producing an acrylic polymer having a reduced low molecular weight component content has not been found so far, which polymer is strongly desired in pressure-sensitive applications such as adhesive tapes.